How would I create an optional expect/actual funct...

# multiplatform

[Sam Stone]

How would I create an optional expect/actual function with a default fallback in commonMain if a platform doesn’t overload it?

I want to use material 3 dynamic colors on android, but default colors on all other platforms, so I have an AppTheme composable, but only want to expect it on android, and otherwise use the preset colors implementation.

[Wout Werkman]

This is currently not supported, you can vote for this issue: https://youtrack.jetbrains.com/issue/KT-20427/Allow-expect-declarations-with-a-default-implementation

[Cheick Kante]

What’s the reason you cannot implement that fallback at the platform level ?
Let’s remember the point of the shared/common code is do/write/architect it once for all platforms and the whole point of expect/actual is for you to implement the part the code that’s platform-dependent while providing your own fallback mechanism (which can be as simple as having a default value) .


Based on what you’re saying “_I want to use material 3 dynamic colors on android, but default colors on all other platforms_”… the compose-multiplatform way of thinking about this is for you to build your ui how you would want it to show on all platforms, and when it comes to the AppTheme part, instead of using the AppTheme from your Common Code, you’ll write your own android specific app-theme (w/ M3).

To guide you with an example:
in commonCode :
@Composable
fun AppTheme(content: @Composable -> Unit )
// no material 3 here

you will call the below function as .. AppTheme { ..content/composables.. }


@Composable
fun AppThemeM3(content: @Composable -> Unit )
in your androidCode:
you will call the below function as .. AppThemeM3 { ..content/composables.. }

[Sam Stone]

@Cheick Kante Except my top-level App composable is the entry point to the app. If I didn’t include AppTheme in App, I would have to wrap the invocation in AppTheme on every platform except Android, and AppThemeM3 on Android. This is the same problem appearing differently. Maybe I could pass a function reference to App with a composable theme and provide AppTheme as a default value, though I don’t know if Compose supports that.